1. Field of the Invention
The present invention relates to a video display device. More particularly, the present invention relates to a display device and a method for driving the same with which image degradation can be reduced that would occur in producing animated images on the display device.
2. Description of the Related Art
Video display devices may generally be categorized as the xe2x80x9choldxe2x80x9d type that allows continuous display of a same video image for a single frame period, or the xe2x80x9cimpulsexe2x80x9d type that allows the video images to be displayed momentarily within a single frame period. Examples of the hold display devices include active matrix liquid crystal displays and organic electroluminescence devices. Examples of the impulse display devices include cathode-ray tubes.
FIGS. 14A and 14B are graphical representations of luminance profiles for a given pixel of the hold and impulse display devices, respectively, plotted against a frame period. As shown in FIG. 14A, the hold display device keeps a constant luminance level over each of the individual frame periods. On the other hand, as shown in FIG. 14B, a peak luminance appears at the beginning of each frame period and the luminance drops down suddenly in the impulse display device.
As apparent from the above, the hold and impulse display devices have different display characteristics. The impulse display devices are said to be superior to the hold display devices in terms of showing animated images. This is described below.
FIGS. 15A and 15B are explanation views showing display characteristics of a hold display device in conjunction with the case where white patterns travel on a black background, in which FIG. 15A shows the displacement of the pattern while FIG. 15B shows a luminance distribution thereof. In FIG. 15A, the ordinate stands for the time; the abscissa stands for the horizontal positions on the display screen; Pn (n is a natural number) represents pixels; and W represents a white pattern of five-pixel wide in the horizontal direction.
With the pattern W moved horizontally by one pixel per a given period of time, a viewer""s eyes follows the pattern W along a line period (from the broken lines V1 to V4) connecting the edges of the pattern W due to a large difference in luminance between the adjacent pixels at each edge of the pattern W.
In FIG. 15A, the broken lines V1 and V2 correspond to a displacement of the viewer""s eyes gazing at the pixel having the smallest value n of the pixels Pn that make up the pattern W. More specifically, the broken lines V1 and V2 correspond to the displacement of the viewer""s eyes at the end and beginning of a given frame period T, respectively, for the pixel in question.
The broken lines V3 and V4 correspond to a displacement of the viewer""s eyes gazing at the pixel having the largest value n of the pixels Pn that make up the pattern W. More specifically, the broken lines V3 and V4 correspond to the displacement of the viewer""s eyes at the end and beginning of a given frame period T, respectively, for the pixel in question.
Thus, the viewer""s eyes gazing at the pattern W moves along the path from the broken line V1 to the broken line V4. However, the video appears as blurred at the regions between the broken lines V1 and V2 as well as between the broken lines V3 and V4 due to the inherent display characteristic of the hold display device that shows the same video image for one frame period as described above with reference to FIG. 14A. The cause of this blurring is described more in detail with reference to FIG. 15B.
In FIG. 15B, portions of the black background are seen in the region left from the broken line V1 and the region right from the broken line V4, and thus the luminance for these regions have the luminance value of zero (0).
There are both light emitting portions and non-light emitting portions in the regions between the broken lines V1 and V2 as well as between the broken lines V3 and V4. The proportion of the light emitting portions becomes larger from the broken line V1 to the broken line V2. The luminance increases accordingly at a constant rate until the luminance reaches the luminance B of the pattern W. Likewise, the proportion of the non-light emitting portions becomes smaller from the broken line V3 to the broken line V4. The luminance decreases accordingly at a constant rate. The luminance B of the pattern W is kept at the region between the broken lines V2 and V3.
As apparent from the above, the edges of the pattern W contains the regions of which luminance changes at a constant rate (between the broken lines V1 and V2 as well as between the broken lines V3 and V4), that is, the regions having the gradient of the luminance curve. This gradient produces the blurry images around the edges. The viewer""s perception of the pattern W is influenced as the pattern W having the five-pixel width L0 is identified to have a larger width L1.
Thus, the hold display device is suffered from significant degradation for animated images. On the contrary, the impulse display device has much less degradation. As described above with reference to FIG. 14B, the luminance drops down suddenly just after the beginning of each frame period. This causes shorter distances between the broken lines V1 and V2 as well as between V3 and V4 (FIG. 15B) as compared with those obtained in the hold display device, resulting in smaller blurry regions. Accordingly, the current accepted theory is that the impulse display devices are superior to the hold display devices in displaying animated images.
Attempts have been made to improve performance of the display for animations by means of making the display characteristics of the hold display devices much closer to those of the impulse display devices. A technique has been proposed as one approach that involves in inserting a black signal (hereinafter, referred to as a xe2x80x9cblack periodxe2x80x9d) into one frame period.
FIG. 16 is a graphical representation of a luminance profile for a given pixel obtained when a black period is inserted into one frame period. As shown in FIG. 16, one frame period is divided into a video display period and the black period. The black periods contribute to making the luminance profiles of the hold display device closer to those of the impulse display device as shown in FIG. 14B.
FIGS. 17A and 17B are explanation views showing display characteristics of a hold display device when a black period is inserted into one frame period, in conjunction with the case where white patterns travel on a black background, in which FIG. 17A shows the displacement of the pattern while FIG. 17B shows a luminance distribution thereof.
In FIG. 17A, the broken lines V1 and V2 correspond to a displacement of the viewer""s eyes gazing at the pixel having the smallest value n of the pixels Pn that make up the pattern W. More specifically, the broken lines V1 and V2 correspond to the displacement of the viewer""s eyes at the beginning of the black period Tb and the video display period Ta, respectively, for the pixel in question. The broken lines V3 and V4 correspond to a displacement of the viewer""s eyes gazing at the pixel having the largest value n of the pixels Pn that make up the pattern W. More specifically, the broken lines V3 and V4 correspond to the displacement of the viewer""s eyes at the beginning of the black period Tb and the video display period Ta, respectively, for the pixel in question.
Comparison between FIGS. 17B and 15B shows that the black periods Tb make it possible to reduce the distance between the broken lines V1 and V2 as well as the distance between the broken lines V3 and V4. The smaller distances result in reduction of size of the regions having the gradient of the luminance curve, which in turn reduces the size of the blurry regions around the edges of the pattern W.
While the video display periods in the frame periods precede the black periods in FIGS. 16 and 17, it should be understood the black periods may precede the video display periods.
With the black periods Tb inserted in the frame periods as shown in FIG. 17B, the luminance of the pattern perceived by the viewer""s eyes is equal to one hundred minus a, all multiplied by B, all divided by one hundred, i.e., B*(100xe2x88x92a)/100, wherein a represents a proportion of the black period Tb in one frame period T. The conventional display devices of the type described have drawbacks in that the black periods Tb inserted into the individual frame periods cause reduction in luminance in proportion to the percentage of the black period Tb in one frame period, as can be seen from the equation.
In addition, the video display periods and the black periods are fixed in order for all frame periods of the video image made up of these periods. Thus, it is impossible to change the order appropriately depending on the type of the video image to be displayed.
The present invention was made with respect to the above and an object thereof is to provide a display device a method for driving the same with which image degradation can be reduced that would occur in producing animated images on the display device.
In order to solve the above-mentioned problems, a display device according to the present invention comprises a display unit on which a video image made up of a plurality of pixels is displayed; a driving unit for driving the display unit; a video signal memory unit for storing a video signal supplied from outside; and a control unit adapted to generate a display signal in accordance with the video signal stored on the video signal memory unit to control driving operation of the driving unit made on the display unit by using the display signal, the display signal being indicative of a video image to be displayed on the display unit for a given frame period, the control unit comprising: calculating means for calculating a value of luminance for a video image to be displayed for a first frame period and a value of luminance for a video image to be displayed for a second frame period subsequent to the first frame period, the values of luminance being calculated for each one of pixels in accordance with the video signal supplied from outside; comparing means for comparing the value of the luminance for the video image to be displayed for the first frame period and the value of the luminance for the video image to be displayed for the second frame period; determining means for determining, according to the comparison result, whether it is necessary to insert a black period into the first frame period to display a black image; and generating means for generating the display signal for the first frame period when the determining means determines that the black period is required, the display signal to be generated having the black period and a video display period for displaying a video image. With this configuration, not all pixels are displayed as black. Instead, only such pixels are displayed as black that are determined to be so in accordance with the change in luminance between the first and second frame periods. Therefore, it is possible to limit the blur of the video images without deteriorating the luminance of the entire display screen.
In the display device according to the present invention, the generating means may be configured to determine the order of the video display period and the black period. With this configuration, the video display periods and the black periods can be aligned in an appropriate order in accordance with the change in luminance between the first and second frame periods. Therefore, image degradation can be reduced more efficiently.
The present inventors have found that the image degradation can be reduced by means of inserting the black period into the first frame period such that the first frame period has the black period and the video image period arranged in this order, when the first frame period has a lower luminance than the second frame period, i.e., when the luminance becomes high. The present inventors have also found that the image degradation can be reduced by means of inserting the black period into the first frame period such that the first frame period has the video image period and the black period arranged in this order, when the first frame period has a higher luminance than the second frame period, i.e., when the luminance becomes low.
Thus, in the display device according to the present invention, the generating means may be configured to generate the display signal such that the video display period precedes the black period, when the value of the luminance for the video image to be displayed for the second frame period is smaller than the value of the luminance for the video image to be displayed for the first frame period and, when a difference is equal to or larger than a predetermined value between the value of the luminance for the video image to be displayed for the first frame period and the value of the luminance for the video image to be displayed for the second frame period. The generating means may be configured to generate the display signal such that the black period precedes the video display period, when the value of the luminance for the video image to be displayed for the second frame period is larger than the value of the luminance for the video image to be displayed for the first frame period and, when a difference is equal to or larger than a predetermined value between the value of the luminance for the video image to be displayed for the first frame period and the value of the luminance for the video image to be displayed for the second frame period.
Furthermore, the display device according to the present invention may comprise a specifying unit for specifying the predetermined value such that the predetermined value falls within a range of luminance available for the display unit. This makes it possible to specify the predetermined value appropriately based on, for example, preference of a user.
In addition, in the display device according to the present invention, the video signal memory unit and the control unit may each comprise a memory area capable of storing the video signal corresponding to one frame. With this configuration, the video signal memory unit is required only to have a memory area for the video signal corresponding to one frame, even when the value of the luminance for the video image for the first frame period is compared with the value of the luminance for the video image for the second frame period, that is, when the video signals corresponding to two frames are compared. Therefore, the configuration becomes cost-effective.
The display device according to the present invention may further comprise a specifying unit for specifying a time length of the black period. This makes it possible to specify a time length of the black period appropriately based on, for example, preference of a user.
In the display device according to the present invention, the video signal supplied from outside represents a gray scale of the video image to be displayed on the display unit and the display device further comprises correspondence information memory unit for storing correspondence information indicative of a correspondence between the gray scale and the luminance. The calculating means may be configured to calculate the value of the luminance for the video image to be displayed for the first frame period and the value of the luminance for the video image to be displayed for the second frame period in accordance with the gray scale represented by the video signal supplied from outside and the correspondence information stored on the correspondence information memory unit. This offers more effective digital signal processing.
In the display device according to the present invention, the control unit may further comprise correcting means for correcting the display signal generated by the generating means such that a value of luminance for a video image to be displayed during the video display period is higher than the value of the luminance for the video image to be displayed for the first frame period. This enhances the edges of the video image presented on the display unit and the blur which otherwise occurs at or around the edges can be reduced.
Furthermore, in the display device according to the present invention, the video signal supplied from outside represents a gray scale of the video image to be displayed on the display unit. The display device further comprises correspondence information memory unit for storing correspondence information indicative of a correspondence between the gray scale and the luminance. The correcting means may be configured to convert a value of luminance for a video image to be displayed during the video display period into a gray scale of the video image in accordance with the correspondence information stored on the correspondence information memory unit. This offers more effective digital signal processing.
In the display de according to the present invention, the correcting means may be configured to convert the value of the luminance for the video image to be displayed during the video display period into a gray scale corresponding to the value of the luminance that is the closest to the value of the luminance for the video image to be displayed during the video display period out of the values of luminance represented by the correspondence information, when the value of the luminance for the video image to be displayed during the video display period is not represented by the correspondence information. With this configuration, it is possible to minimize a difference between the value of the luminance for the video image actually displayed on the display unit and the value of the luminance for the video image to be displayed during the video display period determined by the correction carried out by the correcting means.
Furthermore, in the display device according to the present invention, the correcting means may be configured to convert the value of the luminance for the video image to be displayed during the video display period into a gray scale corresponding to the largest value of the luminance but not larger than the value of the luminance for the video image to be displayed during the video display period out of the values of luminance represented by the correspondence information, when the value of the luminance for the video image to be displayed during the video display period is not represented by the correspondence information. In addition, the correcting means may be configured to convert the value of the luminance for the video image to be displayed during the video display period into a gray scale corresponding to the smallest value of the luminance but larger than the value of the luminance for the video image to be displayed during the video display period out of the values of luminance represented by the correspondence information, when the value of the luminance for the video image to be displayed during the video display period is not represented by the correspondence information. Accordingly, the luminance can be converted more easily into the gray scale.
A method for driving a display device according to the present invention is a method for driving a display device having a display unit on which a video image made up of a plurality of pixels is displayed, the method comprising the steps of: calculating a value of luminance for a video image to be displayed for a first frame period and a value of luminance for a video image to be displayed for a second frame period subsequent to the first frame period, the values of luminance being calculated for each one of pixels in accordance with the video signal supplied from outside; comparing the value of the luminance for the video image to be displayed for the first frame period and the value of the luminance for the video image to be displayed for the second frame period; determining, according to the comparison result, whether it is necessary to insert a black period into the first frame period to display a black image; generating the display signal representing the video image that should be displayed on the display unit, the display signal having a video display period for displaying a video image and a black period for displaying black; and driving a display device to display the video image for the first frame period by using the display signal generated. This makes it possible to limit the blur of the video images without deteriorating the luminance of the entire display screen.
In the method for driving a display device according to the present invention, the step of generating the display signal may determine the order of the video display period and the black period.
In the method for driving a display device according to the present invention, the step of generating the display signal may generate the display signal such that the video display period precedes the black period, when the value of the luminance for the video image to be displayed for the second frame period is smaller than the value of the luminance for the video image to be displayed for the first frame period and, when a difference is equal to or larger than a predetermined value between the value of the luminance for the video image to be displayed for the first frame period and the value of the luminance for the video image to be displayed for the second frame period.
Furthermore, in the method for driving a display device according to the present invention, the step of generating the display signal may generate the display signal such that the black period precedes the video display period, when the value of the luminance for the video image to be displayed for the second frame period is larger than the value of the luminance for the video image to be displayed for the first frame period and, when a difference is equal to or larger than a predetermined value between the value of the luminance for the video image to be displayed for the first frame period and the value of the luminance for the video image to be displayed for the second frame period.
In addition, in the method for driving a display device according to the present invention, the video signal supplied from outside represents a gray scale of the video image to be displayed on the display unit, wherein the calculation step may calculate the value of the luminance for the video image to be displayed for the first frame period and the value of the luminance for the video image to be displayed for the second frame period in accordance with correspondence information indicative of a correspondence between the gray scale and the luminance and the gray scale represented by the video signal supplied from outside.
The method for driving a display device according to the present invention may further comprise the step of correcting the display signal generated such that a value of luminance for a video image to be displayed during the video display period is higher than the value of the luminance for the video image to be displayed for the first frame period.
In addition, in the method for driving a display device according to the present invention, the video signal supplied from outside represents a gray scale of the video image to be displayed on the display unit, wherein the step of correcting the display signal may convert a value of luminance for a video image to be displayed during the video display period into a gray scale of the video image in accordance with correspondence information indicative of a correspondence between the gray scale and the luminance.
In the method for driving a display device according to the present invention, the step of correcting the display signal may convert the value of the luminance for the video image to be displayed during the video display period into a gray scale corresponding to the value of the luminance that is the closest to the value of the luminance for the video image to be displayed during the video display period out of the values of luminance represented by the correspondence information, when the value of the luminance for the video image to be displayed during the video display period is not represented by the correspondence information.
In the method for driving a display device according to the present invention, the step of correcting the display signal may convert the value of the luminance for the video image to be displayed during the video display period into a gray scale corresponding to the largest value of the luminance but not larger than the value of the luminance for the video image to be displayed during the video display period out of the values of luminance represented by the correspondence information, when the value of the luminance for the video image to be displayed during the video display period is not represented by the correspondence information.
Moreover, in the method for driving a display device according to the present invention, the step of correcting the display signal may convert the value of the luminance for the video image to be displayed during the video display period into a gray scale corresponding to the smallest value of the luminance but larger than the value of the luminance for the video image to be displayed during the video display period out of the values of luminance represented by the correspondence information, when the value of the luminance for the video image to be displayed during the video display period is not represented by the correspondence information.
These objects as well as other objects, features and advantages of the invention will become apparent to those skilled in the art from the following description with reference to the accompanying drawings.